Surely if the bolt was loose I'd notice the saddle creaking and moving around?No idea what the post originally was, all the markings/logos on it rubbed off years ago. Have scored an old 2nd-hand post from a mate as a temporary fix. Am planning to go into the LBS for a quick saddle fit and then will look at how much setback I need.

yes you're right, you would expect to notice a loose seat post bolt.Or perhaps you wouldn't,From the article,"Because the lower part of the clamping system is serrated, it is not free to move if the bolt is only slightly loose."

I have had a saddle bolt failure (ala catastrophic, unfortunately for me at the time).I got the bolt checked at Monash Engineering and it showed

1 Work hardening very similar to the failed bolt in the article leading to a critical crack then failure.2 but also when the bolt was measured under magnification it had necked over time (years) from continual tightening.

As you cannot detect necking with the naked eye the moral of the story seems to be to lightly lubricate the bolt at regular periods and possibly consider replacing the bolt every couple of years.

The bolt that failed was an alloy bolt on a Chorus seat post. Steel bolts would be more durable.

I have had 5 or so go over the years... since switching to Thomson posts I have never had a failure. All of my bikes have Thomson posts now, but I find they do have an annoying problem over time of changing shape or wearing ever so slightly around the clamping area... real PITA especially on mtb and CX bikes when they slip down a touch... hence I now have dual clamp seat post clamps... finally I am happy .

I bent/broke several steel bolts before I tried a stainless bolt. Problem solved.

Um, last time I checked, stainless has a lower tensile strength than steel, as well as a greater tendency to gall. Of course the fact that you are clamping aluminium alloy means that the ultimate tensile strength doesn't matter much - if you are breaking a steel bolt, any steel, in an aluminium alloy part, you are using junk bolts. By the way I wouldn't use high tensile bolts for this application either - you do not need the tensile strength & they are more brittle & hence prone to fatigue.

You're probably right. I'm not a metallurgist. But it is obviously a lot stronger than the steel bolts that I had tried in there (including the original one)... It's been there for over 3 years. Admittedly, the steel bolts I used were regular ones from Bunnings. I don't think they were Grade 8, or anywhere near it for that matter.

rkelsen wrote:But it is obviously a lot stronger than the steel bolts that I had tried in there (including the original one)... It's been there for over 3 years. Admittedly, the steel bolts I used were regular ones from Bunnings. I don't think they were Grade 8, or anywhere near it for that matter.

No surprises there. If the steel bolts weren't marked on the head, then they were probably grade 1 or 2. My understanding is stainless is more brittle than steel in general. That is why I was impressed they could use stainless in spokes. But then my DT DB black spokes started to rust (I ride close to salt water) and then I found they could attract a magnet. Obviously not a proper grade of stainless like 304, just "stainless". Impressed no longer.

I'd be surprised if I saw any grade 8 bolts on a bike. High grade bolts are usually found in car suspension parts where the torque settings can be quite high, like 160Nm. Try torquing one of them up when on your back under a car.

__PG__ might be better off with a two bolt clamp design next time. Those bolts should be flexed less.

A failure due to tensile overload has a very typical necking, like this

The pictures I have seen of seatpost bolt failures all show fatigue due to repeated bending, eg likeBreaking at the start of the threads is also typical of this sort of fatigue.

About the only place I can think of that might use grade 8 bolts is for square taper cranks & even then it is probably not necessary. You certainly aren't going to use one in a seat post with an aluminium alloy nut - the nut would strip long before you got anywhere near the required torque for the bolt.

As your experience shows, a stainless bolt is adequate for the task, just that IMO you get a slightly better fatigue margin by using a good quality grade 5 steel bolt

toolonglegs wrote:I have had 5 or so go over the years... since switching to Thomson posts I have never had a failure. All of my bikes have Thomson posts now, but I find they do have an annoying problem over time of changing shape or wearing ever so slightly around the clamping area... real PITA especially on mtb and CX bikes when they slip down a touch... hence I now have dual clamp seat post clamps... finally I am happy .

Hi Toolonglegs,can you possibly link to a pic of a dual clamp seat post clamp?I've added a Thomson to my Surly Ogre having picked up an as-new Masterpiece in the for sale forum.As I'm doing a Wiggle purchase, I was glad to spot a Thomson seat-post spare bolt kit, consisting of bolt, washer and one of those Thomson barrel nuts.I added one to my order which included a bbb torquewrench to assist me not to over tighten said bolts.

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